1. Field of the Invention
The present invention relates to an Orthogonal Frequency Division Multiplexing (OFDM) demodulation apparatus that demodulates OFDM-modulated signals.
2. Description of the Related Art
OFDM modulation, which has a variety of advantages, has been widely used as a multicarrier transmission method for terrestrial digital broadcasting or wireless LAN.
Since OFDM modulation uses a plurality of periodic waveforms, adding a copy of part of an OFDM modulation waveform as a repeated waveform increases resistance in multipath reception. The added waveform part is generally referred to as a guard interval (GI) or a cyclic prefix.
A conventional correlator determines a correlation between a received signal and a part of the received signal corresponding to a guard interval delayed by an effective OFDM symbol. In conventional time synchronization methods, a time position at which the correlation value output from the correlator is maximized in an OFDM symbol period is detected using such a correlator, and a Fast Fourier Transform (FFT) input window position is determined based on the detected time position.
However, there are problems such as a reduction in the correlation value, failing to achieve time synchronization, and degradation of reception characteristics due to variations of transmission path conditions when reception power is low, when the influence of fading and multipath is high, or when a narrow-band noise signal is included in a received band.
Japanese Patent Application Publication No. 2000-165338 describes improvement of reception characteristics by achieving correct symbol synchronization under multipath environments and Japanese Patent Application Publication No. 2004-328583 describes improvement of reception performance by obtaining delayed wave information based on a self-correlation value under a long-delay multipath environment.
However, when OFDM signals are demodulated, it is necessary to perform correct demodulation on the OFDM signals regardless of changes of transmission path states or conditions. Thus, there is a need to realize an effective demodulation device that can perform correct demodulation operations according to changes of transmission path conditions.